Apparatus and method for in-situ chamber cleaning in a compound semiconductor etching system

ABSTRACT

This invention consists of an apparatus and method of permitting automatic, in-situ cleaning of the process chamber and condensation trap of the vacuum pumping system used for etching Gallium Arsenide and other materials producing toxic and condensable etching by-products. This automated cleaning greatly reduces the danger of human exposure to the toxic materials and by-products of the etching. There are two key features. First, the process chamber and condensation trap are designed so that all components are vacuum-sealed to each other (or may be purged with inert gas) such that during wafer processing gas phase species from the process environment cannot condense in narrow spaces between components in the chamber. The surfaces of all parts that come in contact with, and may react to, enchant byproducts, process gases, or any cleaning liquid, are covered with nickel, or Teflon or other highly chemically inert coating. The second feature is that a liquid cleaning cold wall condensation trap is used to capture etching byproducts. The reactor and trap are cleaned separately or simultaneously by injecting water, with or without, liquid cleaner(s) into the process chamber through a set of injection holes. Said liquid cleaner may contain acidic, caustic or other reagents. The wet cleaning step dissolves or converts to soluble form any arsenic (or other toxic) compounds condensed in the process chamber or trap. In addition the flushing action of the liquids aids in the dislodging and removal of any insoluble or low soluble deposits. Liquid cleaning is typically followed with a heated air or nitrogen flow to evaporate any remaining liquid. Said cleaning process may have multiple steps. In a typical clean sequence the chamber and trap are flushed with water-based cleaning compounds that may be acidic or basic. This may be followed by a water-based rinse to rinse away all condensates or remnant compounds from the etching process. The next step may be a rinse with an alcohol or non-aqueous liquid with higher vapor pressure to scavenge or drive out remaining water. And finally, the system may be purged with a drying gas such as nitrogen to remove remnant traces of liquid. The result of such flushing is the virtually complete removal of traces of arsenic and its compounds from the system so as to greatly improve the safety of any ensuing manual chamber clean or replacement of parts. This cleaning process also maintains a more consistent process environment for process stability and reduces particulate that can affect yield.

RELATED APPLICATION

This application is based upon Provisional Application No. 60/472,279, filed May 20, 2003, the priority of which is claimed.

BACKGROUND OF THE INVENTION

The etching of many arsenic containing III-V compound semiconductors, including Gallium Arsenide, produces toxic reaction products such as Arsenic Chloride. These etch reaction products have modest vapor pressures at room temperature and may condense on the walls of the etching reactor and in the vacuum pumping system used to exhaust the etching chamber. This condensate material and materials formed by reaction of condensates with atmospheric gases or moisture are hazardous to the health of the worker who does periodic cleaning or maintenance of the reactor. Previous methods of cleaning reactors used for gallium arsenide etching used gas phase cleaning followed by manual cleaning. While in-situ gas phase cleaning does not expose workers to the chamber interior, manual cleaning does expose workers. Manual cleaning is needed about every several hundred wafers to maintain process integrity. Normally, gas-phase in-situ chamber cleans, which can be very slow and time consuming, still leave hazardous amounts of toxic arsenic compounds condensed in the process chamber and pumping system. Due to this, manual cleaning of the chamber and pumping system for III-V etching systems is done at substantial expense (and some risk) by personnel using hazardous material apparatus. Some new, deep etching applications for the compound semiconductor wafer require etching of several tens of microns of material per wafer rather than the fraction of a micron normally done during wafer fabrication. In this case there will be a great deal more toxic material deposited on the chamber walls than during conventional etching. In addition, there will tend to be large amounts of condensed toxic compounds in vacuum system components such as throttle valves and isolation valves. This will necessitate much more frequent manual cleaning of the chamber and pumping system than is currently performed on such etching chambers—with a substantial increase in operating cost per wafer etched.

One alternative, sometimes used in some etching systems where condensation of reaction products in the process chamber is undesirable, is to heat the chamber walls and internal components. In this case it would also be necessary to heat the proximate components of the pumping system such as isolation valves, throttle valves, and pressure monitors. The required temperature to virtually eliminate all condensation of etch products containing arsenic is rather high—approximately 80 Celsius. Heating such large subsystems to this level is often not possible, and when it is possible, it tends to increase the downtime associated with manual chamber cleans. Downtime is increased because it is difficult to design a process chamber and pumping system components that work reliably over extended periods at such temperatures. One difficulty is the substantial thermal expansion of the metal chamber relative to the support structure and the interfacing wafer handling system. Another is that many seals fail prematurely at elevated temperatures. This elevated temperature also makes working on, or near, the system more difficult and/or more hazardous to all personal. In addition, the arsenic species that do not condense in the reactor will condense in larger quantities further down the pumping path when using a heated chamber design. When these reactants condense in the pumps the pumps become very hard to keep in proper operating condition or require frequent and hazardous cleaning. It would be possible to interpose a cold trap in the pumping line to condense and isolate the toxic compounds, but due to the high etching rate required a high conductance is required. It is very difficult to get the efficiency needed in such a trap while maintaining high gas conductance at low operating pressures. There is a tendency for condensation in the lines, and pump, to be unacceptably high. Therefore, a better, more effective and safer means of virtually completely eliminating such toxic condensates in the etching chamber and pumping system is needed.

SUMMARY

The invention uses an automatic method of wet cleaning both the process chamber and the closely positioned cold trap with its associated plumbing. The invention also provides for the blowout and rapid drying time of the vacuum parts to so that total cleaning time is less then by conventional means.

This improved method and apparatus for the wet cleaning of etch systems having toxic residues, utilizes the physical and chemical cleaning properties of flowing water and water-based chemicals which also can wash away toxic and gritty residues, coupled with the properties of solvents to remove water and other contaminants. The invention provides for normally flat horizontal surfaces to be sloped in order to reduce liquid residue after cleaning, and also provides for liquid shields and appropriately designed O-ring seals to prevent cleaning liquids from entering cracks and other areas that would be difficult to dry.

To eliminate water (and other cleaning fluids) from leaking into the vacuum system, multiple valves are used. The valves isolate water pressure from a water drain, and the water source, as well as from leaking into the vacuum system. Multiple valves are used in series to isolate liquids from vacuum. Since no seal is perfect, an additional valve may be connected to the plumbing line that is located between the two series valves with its exhaust port pointed down so that it vents to the floor of the room (or to a catch tray). Thus, even with very leaky valve seals, no liquid can build up between the two series valves, and thus no liquid can enter the vacuum system through the valve that provides the vacuum seal. Any liquid that exits the plumbing via the valve placed between the two series valves can trigger an alarm to indicate that service is required.

To minimize the drying time of system parts after automatic wet cleaning, water-scavenging solvents (which may include alcohol) may be passed through the system, followed by hot gas (typically air) being blown through the system. Heating the chamber parts can also assist in reducing the time required for complete drying. The heating of the chamber parts also helps reduce the build up of condensates during the etching process. A modest chamber temperature of 50° to 80° C. is all that is required.

Accordingly, several objects and advantages of the invention are to provide for a cleaning method that is safe for personnel as well as faster than combined gas phase cleaning followed by manual cleaning. It is also an object of the invention to provide a better and faster method of etch chamber cleaning that greatly reduces human exposure to arsenic while avoiding the problems associated with virtual leaks when vacuum to liquid seals are required. Still further objects and advantages will become apparent from the study of the following description and accompanying drawings.

DRAWINGS

FIG. 1 is a cross-sectional schematic drawing of a vacuum process chamber with a large vacuum valve and the associated water cleaning inlet valves and water cleaning exhaust valves.

FIG. 2 is a cross-sectional schematic drawing of the chamber lid seal showing the “O” Ring and liquid protection shield.

FIG. 3 is a cross-sectional schematic drawing of a cold trap with large vacuum valves and the associated water cleaning inlet valves and water cleaning exhaust valves.

DETAILED DESCRIPTION OF THE INVENTION

The following description can be applied to any etching system but is most frequently used with a narrow gap parallel plate reactor where the top electrode also serves as the gas supplying showerhead.

FIG. 1 shows a process chamber 1 with lid 2. The lid is curved to allow water flow to provide washing of the lid itself, the chamber walls, the chamber bottom 3 and the lower plumbing lines. Splashing and spraying would allow for the cleaning of electrodes within the reactor (FIG. 1, items 43 and 44). Exhaust line 4 contains a butterfly throttle valve 45, for controlling chamber pressure connects to the main vacuum valve 11. Exhaust line 4 has a sloped bottom 6 to reduce the trapping or puddling of any cleaning liquid. Valve 7 is a vacuum isolation valve used to drain liquid from the chamber and vacuum line 4. Drain line 8 provides means of drainage during cleaning. Valve 9 provides a means of isolating the vacuum system from the backpressure of the drain. Valve 10 is closed when the system is being cleaned so that cleaning water can pass through valve 9 and go to the drain. Valve 10 is open when the system is under vacuum so that any water leaking past valve 9 leaks out to the floor (or tray) and cannot access valve 7, which is a vacuum seal valve.

Water inlet port 17 is connected to valve 16. This valve is opened when water washing is desired. Valve 13 is a vacuum valve and is also open when water washing is desired. Valve 15 is a valve to prevent virtual water leaks, and is closed during water cleaning. This valve is open when valves 13 and 16 are closed to prevent any water leaking past valve 16 from reaching valve 13. Any water leaking past valve 16 will flow out of valve 15 into a catch tray where it can be detected and sound an alarm. Water line 5 connects valve 13 to the relative top of the vacuum chamber.

When liquid washing is in operation, soluble material and some insoluble material is washed off of the chamber lid 2, the top electrode 43, and the bottom electrode 44, the fluid then flows down the chamber sidewalls 1, to the bottom of the sloped chamber 3. All the cleaning solutions then funnel down pumping line 4, and washes and breaks loose solids that are formed on the throttle valve 45, which was left open for cleaning. The concentrated flow is very useful in cleaning this area because the throttle valve is one of the areas that get deposited on the most, and because the moving parts of this valve can easily jam when not kept clean. The cleaning solution then exits the system though open valves 7 and 9.

FIG. 2 shows the vacuum seal, via “O” Ring 20, between the chamber 22 and the chamber lid 18. The small space between chamber lid flange 19 and chamber flange 21 is protected from filling with liquid during automatic wet cleaning by shield 21. As water or water-based chemicals run down the curved chamber lid it comes in contact with the shield and flows down the shield to the side walls of the chamber, thus bypassing the “O” Ring groove and the small space between the lid flange and the chamber flange. For clarity the drawing shows the lid in a position that is slightly open. Water captured in the small space between the flanges would be difficult to remove and be the source of a virtual water leak. The shield prevents water from being captured in this location.

In practice, the chamber plumbing, and the cold trap plumbing, are connected by a high conductance pipe that is as short as possible. Thus the pumping valve 11 shown in FIG. 1 is the same valve show in FIG. 3 as pumping valve 28.

FIG. 3 shows a cold trap 24 with large vacuum valves 28 & 30 and cleaning valves. The cold trap has a liquid nitrogen reservoir 25 for freezing out or condensing most etching gases and byproducts. Rapid cleaning of the trap is accomplished by closing vacuum valves 28 & 30 and allowing the Liquid Nitrogen to pour our of drain 27 (control valve not shown). The trap is then quickly defrosted by flowing water into the trap by opening valves 34 & 36 that admit water form pressurized water line 43. Water and condensate are exited via drain line 42 by opening valves 39 and 41. When the trap is clean, the system can be dried by blowing hot air through the same plumbing lines.

The cold trap 24 does not collect much liquid, as the trap is not level, allowing liquid to efficiently drain to valve 39. Airflow following liquid flow insures that there is no fluid left in the drain tube between the trap and valve 39. In a like manner, the flat surfaces of Liquid Nitrogen reservoir 25 are not level and thus avoid capturing liquid. The plumbing lines 29 & 31 that connect the trap and the large vacuum valves 28 & 30 are also sloped to prevent liquids from being trapped.

As in the description of FIG. 1, when the trap is not being cleaned valves 35 & 40 are opened to prevent any water that leaks past valves 36 & 41 from reaching valve 34 or 39. Thus with no water at the vacuum valve, virtual water leaks are eliminated.

Description of Invention

This invention consists of a new configuration for the etching chamber and cold trap in a high removal rate Ill-V etching system, and a method for their cleaning. This designed apparatus and method of operation assures that virtually all the condensable toxic compounds produced by the etching are trapped, and that these compounds can be readily cleaned with a wet chemical clean. This wet clean must remove substantially all the toxic compounds from the chamber, plumbing, and trap to significantly reduce the safety hazard associated with manual cleaning and maintenance of the chamber and system. This invention can also save time and money by reducing the frequency of manual wet chamber cleans. High rate anisotropic etching of Gallium Arsenide and other III-V materials requires that the chamber and trap have high gas conductance to allow the required high flow rates of etch reactant and reaction product gases at optimum low process pressures. These objectives are accomplished by our design for the configuration and sealing of components within the process chamber and trap, and the means provided within both for wet chemical introduction. By virtue of the design, the wet clean is able to reach and remove all etch products condensed from the gas phase, without degrading the functionality of the chamber. Critical to achieving these objectives, is the method of use where water or other water-based chemicals are used in the cleaning process. As a result, it is one capability of this cleaning system to greatly reduce the required frequency of manual cleaning of the chamber. In some cases the wet clean is able to eliminate the need for manual post-cleaning by adequately removing condensates from critical components. Consequently, etching may be resumed without opening the chamber for cleaning by a human being. This greatly reduces the cost of operation of the system and reduces the time when the system is inoperable.

The design of the chamber, trap, the connection between them, and the connection to the throttle/gate valves are critical so that there are no tight confined spaces in which condensation could occur but the wet clean might not be effective in removing the deposited materials. It is also critical to avoid trapping water or other liquids in confined spaces in the chamber and trap so that they will not affect the etching process. In effect, we describe a seamless interior in the chamber, trap, and valve-sealing surface so that there are no tight confined places accessible to the gas phase or cleaning liquid(s). We show a schematic of the typical etching chamber and valves in FIG. 1, and in FIG. 2 show a large flange area that substantially avoids condensation and liquid. In FIG. 3 we show one embodiment of the cold-trap and valve configuration. In these figures it can be seen that there are typically many spaces where trapping of condensates or liquid might occur. It is also clear that condensation of toxic species will occur on valves that have moving parts, almost certainly compromising their functionality without frequent manual cleaning. However, in this invention the water-based cleaning removes such deposits without manual intervention.

It is well known that supplying water to a vacuum system by conventional means is inadequate in preventing process-damaging water from entering the vacuum system. It is the object of this invention to teach a method of eliminating water leaks into the vacuum system with the use of the design of plumbed valves described herein.

Thus the invention shows for the first time how high conductance pumping can be provided in a system that improves the safety of maintenance people when etching or producing toxic materials in a plasma etcher without the disadvantages of water and liquid contamination.

Because of the large amount of etching and thus large amount of condensate collected, trap cleaning may need to be frequent. Because system uptime is of great importance, downtime due to trap cleaning must be minimized. While trap defrosting typically takes hours, it is an object of this invention to provide trap cleaning and reactivation in less than 30 minutes. This is accomplished by first blowing all coolant from the trap coolant reservoir, and then defrosting the trap with a large volume flow of water. This flow of water and water-based chemicals such as acid or caustic compounds also provides for a substantial portion of the cleaning of the trap. Additional cleaning and removal of remaining water can then be quickly completed with the addition of solvents or high vapor pressure fluids. Finally, drying air is heated to 100 C. by inline electrical heaters and blown through the vacuum section of the trap to remove all moisture. The cleaned trap can then be filled with coolant and its isolation valves opened to the vacuum pumps and process chamber.

It is desirable when designing vacuum systems to avoid water to vacuum seals. This is especially true when grit or deposits can settle on valve seals after a cleaning cycle. It is an object of this invention to provide a method that prevents water leaks into a vacuum caused by grit or deposits that are moved about during a cleaning cycle. The invention uses self-cleaning ball valves to prevent any deposits from compromising the valve seat. As shown in FIG. 3, three two-way valves are connected in such a way that they can supply water to the trap when needed for cleaning but when water is not flowing to the trap there is no vacuum to water seal. In this arrangement if any of the seals leak, only air gets into the vacuum system. Any water getting past a seal goes to a drain or to the room. Detection of any water leak to the room would trigger the system being serviced at a convenient time, as the vacuum system would not be seriously compromised. Water supply and water drain lines would be plumbed in the same configuration. The description mentioned above is only one of several configurations of valves that can accomplish the required task. Only one configuration is shown however one trained in the art would recognize that the same results could be achieved with a smaller number of valves if three or four-way valves were used instead of two-way valves.

Although etching of toxic materials is described in detail it should be obvious to one skilled in the art that all of the same advantages provided by this invention for the etching of toxic materials is also afforded to plasma systems that are depositing toxic materials. For brevity only the etch system is mentioned throughout.

Operation

When the vacuum system is under vacuum and processing work pieces, all valves shown would be closed except pumping valves 28, 29 and water leak prevention valves 15 and 9.

When the system is in need of cleaning, valves 28, 29, 15 and 9 would be closed and cleaning input valves 13 and 17 would be opened along with the throttle valve 45. At the same time liquid-draining valves 7 and 9 would be opened. During this operation liquid would be sprayed and/or flow down the inside of the chamber lid and chamber walls. The liquid shield would prevent liquid from getting into tight spaces that could trap liquid. The chamber, electrodes (not shown), valves and throttling valve would all be cleaned by the action of solvents. In addition non-soluble condensables would be flushed away by the flow of liquid further adding in the cleaning process.

When cleaning was complete heated air would be substituted for the cleaning fluid to dry the system out. Valve positions would remain the same as during liquid cleaning. The flow of hot air would force the liquids to the drains aided by the slopping of otherwise horizontal surfaces. The force of the moving gas pushes the fluids to the drains and the elevated temperature of the gas increases the evaporation rate of the liquids to reduce drying time.

When the system is adequately dried the system is brought back under vacuum by closing valves 13, 17, 7, and 9. While opening valves 14 and 10. The Ln2 trap can then be filled. Valves 28 and 30 can then be opened to pump down the system for processing. Valves having some residue on sealing surfaces will still have acceptable leak rates if self-cleaning ball valves are used.

Thus quick, save, and complete cleaning can be achieved without risk to personal when processing toxic materials such as GaAs. 

1. A plasma etching or deposition system comprising a process chamber, electrodes, vacuum lines with shutoff valves, a valve system to inject cleaning fluid(s) and a valve system to drain fluids from the chamber to be cleaned.
 2. A plasma etching or deposition system comprising a process chamber, electrodes, vacuum lines with shutoff valves making connection to a cold trap for condensation pumping, a valve system to inject cleaning fluid(s) and a valve system to drain fluids from cold traps and pumping lines to be cleaned.
 3. A system as in claim 1 where an additional valve is used to allow liquids to be drained off by a valve open to the room, or catch pan, in order to prevent liquid from reaching the seals of a vacuum valve when the system is under vacuum in order to prevent virtual water or liquid leaks into the process chamber or vacuum system.
 4. A system as in claim 2 where valves are used to allow liquids to be drained off by a valve open to the room, or catch pan, in order to prevent liquid from reaching the seals of a vacuum valve when the system is under vacuum in order to prevent virtual water or liquid leaks into the process chamber or vacuum system.
 5. A system as in claim 1 where liquid(s) are used for cleaning that comprises a water shield to prevent liquids from entering small spaces that would be difficult to dry.
 6. A system as in claim 2 where liquid(s) are used for cleaning that comprises a water shield to prevent liquids from entering small spaces that would be difficult to dry.
 7. A system as in claim 1 where plumbing lines, chamber bottoms, and trap surfaces are all made to be non-horizontal to facilitate good drainage and prevent puddling.
 8. A system as in claim 1 where plumbing lines, chamber bottoms, and trap surfaces are all made to be non-horizontal to facilitate good drainage and prevent puddling.
 9. A system as in claim 1 where the liquid cleaner consists of water and water based chemicals followed by solvents such as alcohol that help remove water from the chamber. 